This is always a problem with those cheap soil humidity sensors, the design is fundamentally flawed; maybe with the hard gold surface treatment (I don't think ENIG is good enough) it could work, but not with exposed copper, tin, or similar. Those cheap boards from china just are not fit for purpose; they should not be used.
You can improve things a little by alternating the polarity you apply to the electrodes in the soil, and by only running it briefly, but those just slow the inevitable. Oh - and they contaminate the soil with dissolved copper...
There are more expensive capacitive sensors that don't rely on the conductivity of the soil; these don't have the problem of electrochemically dissolving themselves in the soil.